These batteries conventionally consist of a plurality of layers comprising an active portion, protective layers, current collector layers and barrier films.
The active portion of the battery comprises a stack of a cathode layer, an electrolyte layer and an anode layer.
This active portion may be obtained using various processes including physical vapor deposition (PVD) techniques such as evaporation or cathode sputtering.
The battery operates as follows: during charging, lithium ions move from the cathode to the anode whereas, during discharging, lithium ions move from the anode to the cathode.
The materials of the cathode, electrolyte and anode layers are chosen depending on the voltage that it is desired to obtain across the terminals of the battery, and depending on the envisioned application.
The cathode layer is made of a material suitable for insertion of lithium ions.
It may especially be a question of lithium cobalt oxide (LiCoO2) or even of lithium titanium oxysulfide (LiTiOS).
In the latter case, the thickness of the layer 13 is comprised between a few hundred nanometers and 10 μm.
Thus, by way of example, a TiOS layer may be formed on the surface of the cathode current collector layer, a thin-film of lithium then being deposited on the TiOS layer using a PVD process. The lithium then naturally diffuses into the TiOS layer, which is converted into a layer of LiTiOS.
Another process is described in document EP 2 320 502, according to which the insertion of lithium ions is obtained from lithium deposited on the anode layer. It is the presence of a short-circuit between the anode and cathode layers that allows the lithium to migrate and Li+ ions to penetrate into the cathode layer. The anode and cathode layers are separated once the migration of the lithium has terminated.
Thus, this process allows the cathode layer to be lithiated after the electrolyte and anode layers have been deposited.
This has an advantage as regards the formation of interfaces between the electrolyte layer and the cathode and anode layers. Specifically, lithium is a very reactive metal that is liable to disrupt the formation of these interfaces, this possibly leading the layers to adhere poorly at these interfaces.
However, the process described in document EP 2 320 502 has many drawbacks.
Specifically, this process is relatively complex to implement since it requires the various layers of the stack to be deposited locally and, in particular, a layer of complex shape, comprising a plurality of portions, to be formed on the substrate. One of these portions forms a cathode current collector layer that receives the stack. The others form pads intended to receive cathode and anode connections, which are connected to one another by a conductive strip.
Using a masking technique to localize the deposit limits the dimensional resolution of the batteries because it requires the various levels of active layers to be aligned with one another in each step employing a mask.
This manufacturing process is therefore of a complexity that will increase with industrialization and as the size of the batteries decreases.
The anode and cathode layers are short-circuited on the one hand using the conductive strip connecting the two pads, said strip being produced at the start of the battery manufacturing process, and on the other hand using the electronic conductivity of the anode layer, the latter becoming electronically conductive during the deposition of the lithium.
Thus, the electronic conductivity of the anode layer decreases as lithium diffuses into the TiOS layer. Thus, the effect of the short-circuit gradually decreases and the battery cannot therefore be completely discharged in this way.
Lastly, this manufacturing process requires the short-circuit to be opened in each battery before the batteries are separated from one another, this constituting an additional step of the manufacturing process.